Parse documents on a Celery parsing worker via a read_document tool

Replace the sandbox Docling extractor with read_document, backed by the in-process
backend parser (the same one ingestion uses) and offloaded to a dedicated
'parsing' Celery queue so it can run on GPU-capable workers with predictable RAM.
The tool resolves the input ref under the run-scoped gate, enqueues the parse,
and awaits it with a timeout (degrading to an error rather than hanging); the
worker independently re-resolves the artifact through the same gate and never
trusts a raw path. Untrusted files get the upload path's safeguards (extension
whitelist, size cap, sanitized temp file, cleanup). Options: output
(markdown/text/structured/chunks), ocr, pages, engine, max_chars, include_tables,
persist, json_schema. The workflow native-file 'extract' fallback now uses the
same worker path, so document parsing no longer needs the sandbox and works on
every backend.

Also fixes the branch's periodic-task test (the sandbox reaper made it 12) and
points the dev and e2e Celery workers at the parsing queue.

tests/api/user/test_tasks.py

@@ -276,7 +276,7 @@ class TestSetupPeriodicTasks:
 
         setup_periodic_tasks(sender)
 
-        assert sender.add_periodic_task.call_count == 11
+        assert sender.add_periodic_task.call_count == 12
 
         calls = sender.add_periodic_task.call_args_list
 
@@ -308,6 +308,9 @@ class TestSetupPeriodicTasks:
         # schedule runs cleanup (24h)
         assert calls[10][0][0] == timedelta(hours=24)
         assert calls[10][1].get("name") == "cleanup-schedule-runs"
+        # sandbox session reaper (60s)
+        assert calls[11][0][0] == timedelta(seconds=60)
+        assert calls[11][1].get("name") == "reap-sandbox-sessions"
 
 
 class TestMcpOauthTask: